The present invention is directed to a novel process for upgrading Fischer-Tropsch products, and more particularly to a novel process for upgrading Fischer-Tropsch products, thereby forming light olefins.
Fischer-Tropsch products typically are rich in linear paraffins, but often are contaminated with oxygenates, olefins, and on occasion traces of nitrogen compounds. These contaminants are generally not desirable in salable products, such as diesel fuel, paraffinic naphtha, jet fuel, liquefied petroleum gas (LPG), solvents, aromatics, lube base stock, and combinations thereof, so they must be removed by upgrading processes.
The most common upgrading process is hydroprocessing. Hydroprocessing is a general term meant to include more specific technologies such as hydrotreating, hydrocracking, hydroisomerization, reforming, and hydrodewaxing. Hydroprocessing in general converts oxygenates and olefins into additional paraffins and removes heteroatom compounds, such as nitrogen compounds. Hydroprocessing is performed by contacting a feed over a catalyst that contains a metal in the presence of hydrogen at pressures and temperatures greater than ambient. While olefins in Fischer-Tropsch products can contribute to the formation of carbon deposits on hydroprocessing catalysts, thus poisoning them, these catalysts can be regenerated by burning the carbon deposits. However, regeneration typically operates at elevated pressures. Therefore, regeneration requires expensive facilities to conduct in-situ regeneration, or facilities to load and unload the catalysts from the hydroprocessing reactor and regenerate the catalysts off-site.
In addition, almost all hydroprocessing technologies require the use of hydrogen gas as a reactant. This hydrogen gas is expensive to obtain and to store. Reforming is the only hydroprocessing technology that does not require the continued consumption of expensive hydrogen gas. Although reforming does not require the consumption of hydrogen, reforming does require pretreatment of the feed by hydrotreating to remove any oxygenates and heteroatom compounds, such as nitrogen compounds. When reforming is used to convert Fischer-Tropsch products, typically C6-C10 products, to benzene, toluene, xylene, and other aromatics, the catalyst typically comprises platinum on an alumina support in the presence of a halogen, commonly chloride. The chloride is essential to operation of the reforming catalyst. However, if the feed contains oxygenates and any residual nitrogen compounds, the oxygenates strip the chloride from the catalyst, forming water, and the nitrogen compounds react with the chloride forming volatile ammonium chloride. The volatile ammonium chloride leaves the reactor and can cause corrosion problems in downstream equipment. Therefore, pretreatment of the feed to remove oxygenates and nitrogen compounds is especially important when reforming is used as the upgrading process. Typically a hydroprocessing unit, such as a hydrotreater, upstream of the reformer, is used to accomplish this pretreatment.
Upgrading processes for hydrocarbon feeds, including processes for petroleum feeds and processes for Fischer-Tropsch feeds, are known in the art. By way of example, U.S. Pat. Nos. 4,171,257 and 4,251,348 relate to processes for upgrading a petroleum distillate feed. In this upgrading process, the petroleum feed, containing a significant content of normal paraffins, is dewaxed with ZSM-5 zeolite, and the effluent product stream is fractionated producing a C3-C4 olefin product fraction.
U.S. Pat. No. 4,234,412 relates to a process for upgrading a reaction product obtained in a Fischer-Tropsch hydrocarbon synthesis. The process comprises separating the product into at least one of a light boiling fraction and/or heavy boiling fraction and contacting the fraction(s) with certain crystalline silicates to obtain an aromatic gasoline and/or a fuel oil having a lowered pour point.
U.S. Pat. Nos. 6,455,750 and 6,069,287 relate to a process for producing light olefins from a catalytically cracked or thermally cracked naphtha stream. The cracked naphtha, which contains 10 to 30 wt % paraffins and 20 to 70 wt % olefins, is cracked with a catalyst containing a crystalline zeolite having an average pore diameter less than about 0.7 nanometers at reaction conditions.
U.S. Pat. No. 4,361,503 relates to an improved process for converting synthesis gas to hydrocarbon mixtures using an improved catalyst composition. The catalyst comprises an iron-containing, Fischer-Tropsch catalyst and a crystalline zeolite having a silica-to-alumina ratio of greater than 200 (including zeolites containing essentially no alumina) and an (R2O+M2/nO):SiO2 ratio of less than 1.1:1, where M is a metal other than a metal of Group IIIA, n is the valence of the metal, and R is an alkyl ammonium radical. This process using the above catalyst composition increases the selectivity to olefinic naphtha products.
PCT application WO 00/53695 relates to an environmentally friendly gas conversion process, which produces and disposes of ammonia in the process. The gas conversion process includes producing a synthesis gas, which contains ammonia and hydrogen cyanide. The synthesis gas is used to form hydrocarbons by reacting the hydrogen and carbon monoxide in the gas in the presence of a hydrocarbon synthesis catalyst. However, the synthesis gas reversibly deactivates the catalyst due to the presence of the ammonia and hydrogen cyanide in the gas. The catalyst is rejuvenated with a gas comprising hydrogen producing an ammonia containing rejuvenation offgas. The ammonia is dissolved out of the offgas with water and then stripped out of the water with the hydrocarbon feed to the synthesis gas generator and into the generator where it is consumed. This process can contribute to the formation of nitrogen in products from the Fischer-Tropsch process.
European patent EP 0 757969B1 relates to a process for the removal of hydrogen cyanide, HCN, from synthesis gas. HCN is a poison for Fischer-Tropsch hydrocarbon synthesis processes. The HCN concentration of HCN containing synthesis gas streams is reduced by treatment with a Group IVA metal oxide and optionally containing a Group IIB, Group VA, or Group VIA metal or metals, at reaction conditions preferably suppressing Fischer-Tropsch activity. This process also can contribute to the formation of nitrogen in products from the Fischer-Tropsch process.
U.S. patent application Ser. No. 09/758,750 relates to a process for upgrading nitrogen-containing Fischer-Tropsch products using hydroprocessing. U.S. patent application Ser. No. 09/758,751 relates to the use of chemical analysis of Fischer-Tropsch waxes, in particular, the determination of heteroatom content, including nitrogen, in Fischer-Tropsch waxes.
Accordingly, efficient and inexpensive processes to reduce or eliminate olefin, oxygenate, and heteroatom compound impurities in Fischer-Tropsch products are desired, while at the same time converting as much of the Fischer-Tropsch products to form valuable products as is possible. Therefore, efficient and inexpensive processes to convert the olefin and oxygenate impurities to form more valuable products are also desired.
A process for upgrading a Fischer-Tropsch product comprising paraffins, oxygenates, and C6+ olefins is disclosed. The process includes contacting the Fischer-Tropsch product with an acidic olefin cracking catalyst to convert the oxygenates and C6+ olefins to form light olefins. The contacting conditions include a temperature in the range of about 500xc2x0 F. to 850xc2x0 F., a pressure below 1000 psig, and a liquid hourly space velocity in the range of from about 1 to 20 hrxe2x88x921. The process further includes recovering the Fischer-Tropsch product comprising unreacted paraffins, and recovering the light olefins.
In another embodiment, a process for producing saleable products from a Fischer-Tropsch product stream is disclosed. The process includes producing a Fischer-Tropsch product stream comprising paraffins, oxygenates, and C6+ olefins. The Fischer-Tropsch product stream is contacted with an acidic olefin cracking catalyst to convert the oxygenates and C6+ olefins to form light olefins, providing a stream comprising light olefins and unreacted paraffins. The contacting conditions include a temperature in the range of about 500xc2x0 F. to 850xc2x0 F., a pressure below 1000 psig, and a liquid hourly space velocity in the range of from about 1 to 20 hrxe2x88x921. The unreacted paraffins and light olefins are separated. The light olefins are recovered and a salable product is produced from the light olefins. The salable product produced from the light olefins may be commercial grade propylene, high octane gasoline blend components, polypropylene, polyisobutylene, isooctane, cumene, isopropyl alcohol, tertiary butyl alcohol, methyl tertiary-butyl ether, tertiary-amyl methyl ether, ethyl tertiary-butyl ether, and tertiary-amyl ethyl ether, and combinations thereof. The unreacted paraffins are recovered and a salable product is produced from the unreacted paraffins. The salable product produced from the unreacted paraffins may be diesel fuel, paraffinic naphtha, jet fuel, liquefied petroleum gas, solvents, lube base stock, and combinations thereof.
In yet another embodiment, a process for upgrading a Fischer-Tropsch product comprising paraffins, oxygenates, and C6+ olefins is disclosed. The process includes contacting the Fischer-Tropsch product with an acidic olefin cracking catalyst to convert the oxygenates and C6+ olefins to form light olefins, providing an effluent comprising unreacted paraffins and light olefins. The contacting conditions include a temperature in the range of about 500xc2x0 F. to 850xc2x0 F., a pressure below 1000 psig, and a liquid hourly space velocity in the range of from about 1 to 20 hrxe2x88x921. The effluent is cooled to convert the unreacted paraffins into a liquid and the unreacted liquid paraffins are recovered. The process includes further cooling the effluent to convert at least a portion of the light olefins into a liquid and recovering the light olefins.